In the production of oil or gas from an subsurface formation, steel liner pipes with multiple longitudinal slots (“slotted liners”) are commonly installed in both vertical and horizontal wells to allow oil or gas present in the formation to enter the wells, whereupon the oil or gas can then be pumped or otherwise lifted to the surface for processing. The slots must be narrow enough to prevent significant amounts of formation materials from entering and clogging up the well and associated equipment such as pumps. For wells installed in formations containing fine-grained materials, liner slot width may need to be as narrow as 0.04 inches (1.0 millimeter) or even considerably less. The slots must be long enough and numerous enough to allow for effective flow into the liner, without reducing the liner's structural strength below safe levels. The liner's structural strength (especially its flexural strength) is particularly important for horizontal wells, in which the liner must retain sufficient strength to be bent through transition sections between vertical and horizontal wellbores without fracture or excessive plastic deformation.
The slots may be of any convenient length, but they are typically in the range of 3 to 4 inches (75 to 100 millimeters) long. They are usually arrayed at uniform spacing about the circumference of the pipe, at radial intervals as low as 5 degrees. They are commonly cut into the liner sidewall using narrow circular slitting blades. One known method uses a “gang mill” fitted with multiple slitting blades radially oriented on planes passing through the longitudinal axis of the liner. As the liner is moved longitudinally relative to the gang mill, the blades are deployed so as to cut slots of desired length through the liner sidewall.
Rather than making perfectly clean cuts, the slitting blades tend to leave jagged burrs or tendril-like “wickers” where the slots intercept the interior surface of the liner. These burrs and wickers are undesirable for a variety of reasons, so the production of slotted liners typically includes steps to remove them, but known methods of doing so are not entirely satisfactory. One common method is to run a device commonly called a “stinger” through the slotted liner. The stinger has multiple rotating blades disposed such that they will essentially scrape the interior perimeter of the liner as the stinger passes through. The intent is that the rotating blades will cut off the wickers, which can then be removed from the liner by compressed air or other means.
However, this method has proved to be only partially effective, because the scraping blades tend to bend the burrs and wickers and push them back across or into the slots, causing a direct reduction in the open slot area available for passage of oil into the liner. This problem is particularly evident for slot widths of 0.04 inches (1 mm) and less. The effective slot area tends to become further reduced when the liner is placed in service, because foreign materials entering the slots build up on the bent-back wickers, causing the slots to become partially or totally plugged.
Other mechanical methods, such as honing or burnishing, have been used in an attempt to polish the wickers down. However, these methods have similar drawbacks, in that they tend to simply brush some or all of the wicker metal back into the slots.
When using known de-burring methods having such significant drawbacks, it may be necessary to allow for slot plugging by providing a greater amount of slotting than might otherwise be required. It has been observed that slot plugging can reduce the effective permeability of a slotted liner by as much as 40% to 60%, so in order to obtain a desired permeability, liners may have to have a slotted area up to or more than twice as large as the area theoretically required for a given application. Such extra slotting obviously increases liner fabrication cost. It also decreases the structural strength of the liner, possibly entailing the use of liners with greater wall thickness, thus increasing the total cost of the slotted liner even further.
In addition to the foregoing problems, wickers or any other material left inside slotted liners can damage or interfere with expensive down-hole tools used in well-servicing operations.
A possible alternative approach to wicker and burr removal would be thermal de-burring; i.e., exposing the wickers and burrs to a high-temperature flame. It is well known that burrs of steel or other materials can be burned off and effectively incinerated if subjected to a sufficiently hot flame. This would facilitate very effective removal of burrs from a slotted liner, as it would be fairly simple to remove the residue from the process (i.e., oxides) using compressed air, high-pressure water blasting, or other conventional means.
For the type of steel commonly used for slotted liners, effective use of this method would require heating the burrs to temperatures in the range of 6000° F. (3316° C.). At the same time, though, care would have to be taken to ensure that the temperature of the main body of the liner does not become excessive, in order to prevent undesirable metallurgical changes in the parent metal. This would not be overly difficult if the high-temperature flame could be effectively focused or concentrated on the burrs and not on the main body of the liner, because the much larger mass of the liner (i.e., compared to the mass of the burrs) would allow efficient dissipation of the heat applied to the burrs through conduction, without excessive temperature build-up in the parent material. However, it is virtually impossible to direct a flame toward burrs inside a steel liner without exposing the main body of the liner to the flame. Furthermore, the inventor has observed that when conventional flame sources such as acetylene torches are used in an attempt to heat burrs inside a slotted liner to temperatures sufficient to achieve vaporization, the flame must dwell upon the burrs for so long that excessive localized heating of the parent metal is unavoidable. The prior art appears to disclose no solution to this problem.
For the foregoing reasons, there is a need for wicker-removal and de-burring apparatus and methods that can remove burrs and wickers from slotted metal liners with substantially greater effectiveness than known apparatus and methods. In particular, there is a need for such apparatus and methods that can remove burrs and wickers by exposure to an oxidizing flame, without raising the temperature of the adjacent parent metal so high as to cause metallurgical changes or other undesirable effects. The present invention is directed to these needs.